JP2524827B2 - Laminated thin film body and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention forms an ultrathin film of MgO on the surface of a YBa ₂ Cu ₃ O _7-X thin film, which is attracting attention as showing superconductivity in the vicinity of 90K. And a method suitable for producing the laminated thin film body.

Not only YBa ₂ Cu ₃ O _7-X , but superconductors are expected to have new applications such as Josephson tunnel type devices.

The Josephson tunnel type device requires an ultrathin film for insulation of 30 Å or less in order to make a tunnel junction with a superconductor, and in order to form such a junction, a superconducting thin film or a junction with excellent surface smoothness is required. It is necessary to fabricate an ultra thin film for insulation. The thickness of the insulating ultrathin film used for bonding is limited by the length of the coherence of the superconducting thin film.
The coherence length in the direction perpendicular to the (001) plane is 4 to 7Å
It is said that it is 15 to 30Å in the parallel direction.

Therefore, the thickness of the insulating ultra-thin film used for bonding depends on what kind of superconductor is used as the material to be bonded, and the direction perpendicular to the (001) plane of the superconductor is the insulating Regarding the relationship in the thickness direction of the thin film, the thickness of the ultra thin film for insulation must be 10Å or less.

On the other hand, in the relationship that the direction parallel to the (001) plane of the superconductor is the thickness direction of the insulating ultra-thin film, the thickness of the insulating ultra-thin film may be several tens of liters, forming a tunnel junction. Is convenient for.

The present invention relates to the former of the above,
Specifically, a single crystal Y with the (001) plane parallel to the film plane.
Ba ₂ Cu ₃ O _7-X on the surface of the thin film, MgO less than 10 Å without interruption
The present invention relates to a laminated thin film body obtained by epitaxially growing an ultrathin film and a method suitable for manufacturing the laminated thin film body.

[Conventional technology]

Single crystal YBa ₂ Cu ₃ O with (001) plane parallel to the film plane
_The method for manufacturing the _7-X thin film itself has already been applied as a result of the present inventors.

On the other hand, the results of research on epitaxial growth of (110) MgO on (110) NbBaCuO by sputtering are as follows.
Autumn Meeting 49th Annual Meeting of the Japan Society of Applied Physics Proceedings 1st Volume No. 1
Published on page 106.

[Problems to be Solved by the Invention]

However, it is extremely difficult to form an ultrathin film for insulation on a high-temperature superconducting thin film, and conventionally, even a thin film for insulation of several pick-up order could not be obtained as a completely uninterrupted film.

This is partly because it was difficult to provide a high-temperature superconducting thin film as a base material that could be said to be substantially a single crystal, and epitaxial growth of an ultra-thin insulating film on it was difficult to achieve. However, this problem regarding the high temperature superconducting thin film has been almost solved by the method proposed by the present inventors.

However, even though many studies have been made on the formation of the insulating ultra-thin film on the high-temperature superconducting thin film thereafter, the continuous formation of the insulating ultra-thin film has not been successful.

That is, the same is true of the example of epitaxial growth of (110) MgO on (110) NbBaCuO described above, but in the past, even though the insulating thin film was grown epitaxially, it was microscopic. In reality, it is nothing but the fact that the crystals only grow in a state of scattered islands.Therefore, as the thin film becomes an ultra-thin film, In some parts, discontinuous parts of the film were formed everywhere.

In view of the above, the inventors of the present invention have formed a single crystal YBa ₂ Cu ₃ O _7-X thin film whose (001) plane is parallel to the film surface, and an ultrathin insulating film made of MgO for each atomic layer. The present invention has been achieved as a result of earnest research with the goal of making it possible to provide an ultra-thin film without interruption by sequentially growing the films.

[Means for Solving the Problem] and [Action] That is, the present invention is directed to a single crystal YBa ₂ Cu ₃ O having a (001) plane parallel to the film surface.
A laminated thin film body characterized in that an ultra-thin MgO thin film having a thickness of 10 Å or less is formed on the surface of a _7-X thin film, the (001) plane being parallel to the film surface.

The laminated thin film body as described above, wherein the ultrathin MgO film is formed as a single crystal.

On the surface of a single crystal YBa ₂ Cu ₃ O _7-X thin film with the (001) plane parallel to the film surface formed on the evaporation substrate in the vacuum evaporation tank, in the presence of a trace amount of oxygen, the evaporation substrate At a temperature of less than 500 ° C. to evaporate Mg at a rate of 2Å / sec or less to form a MgO thin film, which is a method for producing a laminated thin film body.

The relations of the above-mentioned inventions are briefly as follows.

That is, the inventions of and are related to the laminated thin film body which is considered to be necessary as a pre-stage when used as the above-mentioned Josephson tunnel type element, and the invention of
It is possible to gradually grow MgO on the YBa ₂ Cu ₃ O _7-X plane in atomic layer units, and it is also possible to form an uninterrupted MgO ultra-thin film and a MgO thin film with excellent surface smoothness. The present invention relates to a method invention.

Here, the manufacturing method of the single crystal YBa ₂ Cu ₃ O _7-X thin film in which the (001) plane is parallel to the film surface, which is common to all the above-mentioned inventions, is as previously filed. Specifically, the following method (a) or (b) is used.

(B) Oxygen gas is injected from the vicinity of the surface of the vapor deposition substrate in the vacuum vapor deposition tank to create an oxygen atmosphere with a relatively high pressure only near the vapor deposition substrate, and each of the Y, Ba, and Cu metals is vaporized separately. A method in which the atomic ratio of Y: Ba: Cu from the source is controlled to be approximately 1: 2: 3 and vaporized simultaneously onto the substrate.

(B) Oxygen gas is injected from the vicinity of the surface of the vapor deposition substrate in the vacuum vapor deposition tank to create an oxygen atmosphere of relatively high pressure only near the vapor deposition substrate, while plasma is generated in the vacuum vapor deposition tank. , Ba, Cu metals from different evaporation sources Y: Ba: Cu
A method of simultaneously evaporating onto the substrate while controlling so that the atomic ratio of is about 1: 2: 3.

YBa ₂ Cu ₃ O produced by the above method (a) or (b)
_To obtain a _7-X thin film as a single crystal whose (001) plane is parallel to the film plane, SrTiO ₃ , MgO, C ₀ O, NiO
Etc., and the (001) plane is used as the substrate surface.

Thus, the YBa ₂ Cu ₃ O _7-X single crystal thin film is
In order to obtain a plane parallel to the film plane, it is necessary to use a single crystal whose surface is a (001) plane at least as a vapor deposition substrate. In addition to that, YBa ₂ Cu ₃ O _7-X In order to obtain the thin film as a substantially single crystal as a whole, it is necessary to evaporate the metal on the vapor deposition substrate heated to 500 ° C. or higher, more preferably 520 ° C. or higher.

Furthermore, the following operations are performed in the production of the YBa ₂ Cu ₃ O _7-X single crystal thin film.

First, the vacuum vapor deposition tank was initially set to a high vacuum of, for example, about 10 ^-6 Torr, and then a small amount of oxygen gas was continuously jetted from the vicinity of the vapor deposition substrate toward the surface of the substrate, The oxygen gas pressure is increased to 10 ^-2 to 10 ^-1 Torr only in the vicinity of the surface, while the gas in the vacuum deposition tank is continuously exhausted from the appropriate location in the vacuum deposition tank, and most of the vacuum deposition except the vicinity of the deposition substrate is performed. The oxygen gas pressure in the tank is set to 10 ^-5 to 10 ^-3 Torr.

By the first means, the upper limit of the oxygen gas pressure in the portion other than the vicinity of the vapor deposition substrate in the vacuum vapor deposition tank is set to 10 ^-3 Torr, which deteriorates Y, Ba, Cu in the evaporation source in the same vapor deposition tank. This is because the evaporation can be carried out smoothly. On the other hand, the lower limit of 10 ^-5 Torr is the lower limit of the gas pressure required when plasma is generated, and has no particular technical meaning when plasma is not used.

Further, the reason for increasing the oxygen gas pressure only near the vapor deposition substrate by the first means is to oxidize Cu to Cu ²⁺ to Cu ^{3+. At} the oxygen gas pressure of 10 ^-3 Torr or less, Cu is This is because Cu ^{2 to} Cu ³ cannot be oxidized.

The plasma can be generated by placing a high frequency coil between the evaporation source and the vapor deposition substrate and causing high frequency oscillation between the plasma deposition chamber and the chamber wall of the vacuum vapor deposition tank. On the other hand, it is desirable in terms of improving reaction activity, but if the generation is strong, it will cause adverse effects such as attacking the target object during generation, so the power used for plasma generation is 50 W to 500 W, preferably around 100 W. .

Secondly, for the evaporation of Y, Ba and Cu, an electron beam may be used for Y and Ba, and electric resistance heating may be used for Cu.

Then, in the evaporation of the metal by these evaporation means, by the electric power determined by the preliminary experiment in the vacuum deposition tank performed prior to the implementation, the evaporation amount of Y: Ba: Cu becomes about 1: 2: 3. You can set it to.

In other words, Y, B
The degree of evaporation of each metal of a and Cu to form a vapor-deposited film of Y ₂ O ₃ , BaO, and CuO per unit time under the electric power condition applied to the vaporization source It is possible to measure each metal with a film thickness meter installed near the substrate and determine the evaporation amount at the time of implementation by the amount of electric power applied to the evaporation source.

In order to epitaxially form MgO on the surface of the single crystal YBa ₂ Cu ₃ O _7-X thin film having the (001) plane parallel to the film surface produced as described above, YBa ₂ Cu It is possible to evaporate Mg under the same conditions as ₃ O _7-X .

However, according to the experiments by the present inventors, in forming the MgO thin film, the deposition substrate temperature is 500 ° C. or higher, more preferably 520 ° C. or higher, and YBa ₂ Cu ₃ O _7-X is a deposition substrate temperature at which it is easy to grow epitaxially. It was found that uniform growth of MgO in atomic layer units could not be expected under the same conditions as above, and under the above conditions, MgO easily grows like islands as in the conventional case, and at most 30 Å It was found that if the film was not formed into a thin film, a discontinuous portion would occur in the film.

In forming the MgO thin film according to the present invention, the deposition substrate temperature is 500 ° C.
The reason why the amount is less than the above is due to the above-mentioned reason, and as will be apparent from the examples described below, even if the deposition substrate temperature is appropriate, a thin film of MgO having a surprising thinness of 2 to 3 atomic layers is not interrupted. Can be formed. Moreover,
This ultrathin MgO film has not only the (001) plane that is parallel to the film plane but also a single crystal as a whole. It is also very convenient for epitaxially growing good quality YBa ₂ Cu ₃ O _7-X whose plane is parallel to the film surface.

In forming the MgO thin film, it was decided to evaporate Mg toward the vapor deposition substrate at a rate of 2 Å / sec or less.
This is because if the evaporation rate is too fast, MgO easily crystallizes in the form of islands, and if the MgO thin film is an ultra-thin film, the film tends to break.

The evaporated Mg is combined with oxygen in the vacuum deposition tank and deposited as MgO on the deposition substrate.
Since a high oxygen concentration is not required locally as in the case of Cu oxidation, the oxygen pressure in the entire vacuum deposition may be about 10 ⁻⁴ Torr.

〔Example〕

Evacuate the vacuum evaporation tank (750φ x 1000h) to 10 ^-6 Torr with an oil diffusion pump.

Sr with a polished surface is used as a vapor deposition substrate for growing thin films.
TiO ₃ single crystal was used with its surface being (001) plane (10 ^mm × 10 ^mm ), which was heated to 650 ° C by a W-ray heater.
Heat up to and hold at this temperature.

Insert the oxygen gas injection nozzle into the donut-shaped oxygen diffusion chamber that surrounds the outer peripheral edge of the vapor deposition substrate, and the oxygen ejected from the oxygen nozzle once diffuses in the oxygen diffusion chamber, and then is provided on the inner peripheral surface of the oxygen diffusion chamber. The thin layer is ejected from the gap along the surface of the vapor deposition substrate. At this time, the gas pressure rises to 10 ^-2 to 10 ^-1 Torr only near the vapor deposition substrate, but only reaches 10 ^-4 Torr near the evaporation source away from the vapor deposition substrate.

Evaporation rates of metal Y, Ba and Cu from independent evaporation sources such that the atomic ratio becomes 1: 2: 3 on the deposition substrate (for example, Y
・ ・ ・ 1Å / sec, Ba ・ ・ ・ 2.3Å / sec, Cu ・ ・ ・ 1.7Å / se
Evaporate in c). Further, a high frequency coil is placed between the evaporation source and the vapor deposition substrate to generate high frequency oscillation at 100 W to generate oxygen plasma and activate the vaporized metal to accelerate the reaction on the vapor deposition substrate.

As described above, a thin film of YBa ₂ Cu ₃ O _{7-X having} a film thickness of 1000 Å was obtained.

Next, the temperature of the vapor deposition substrate was lowered to 305 ° C., the supply of oxygen to the surface of the vapor deposition substrate was stopped, and the oxygen pressure in the vacuum vapor deposition tank was adjusted to the level of 10 ⁻⁴ Torr, and then YBa ₂ Cu ₃ O obtained above was obtained. Mg was evaporated on the _7-X thin film at an evaporation rate of 1Å / sec to increase the film thickness.

In the above implementation, a backscattered electron diffraction image of a YBa ₂ Cu ₃ O _7-X thin film with a film thickness of 1000 Å and a backscattered electron diffraction image of an MgO thin film formed on the YBa ₂ Cu ₃ O _7-X thin film were obtained. At the time of, the (001) plane of YBa ₂ Cu ₃ O _7-X was grown epitaxially with the film plane parallel to the film plane, and was essentially a single crystal, and the MgO film thickness was 10 Å or less. It was confirmed that even in the case of (1), it grew evenly without growing in an island shape, and epitaxially grew with the (001) plane parallel to the film surface, and was close to a single crystal.

Moreover, the formation of a YBa ₂ Cu ₃ O _7-X thin film on the above MgO thin film was tried under the same conditions as the above-mentioned thin film, and the thin film was epitaxially formed with its (001) plane parallel to the film plane. It was also confirmed that the film grew to a substantially single crystal film.

As evaporation sources, electron beam evaporation and Cu
For each, resistance heating evaporation was used. For Mg, electron beam evaporation was used.

Next, the evaporation method will be described for each.

Y: 50 g of a metal ingot (99.9%) was used, and this was put in a water-cooled crucible and an electron beam was applied to the metal at an accelerating voltage of 5 KV and a filament current of 400 mA to evaporate.

As with Ba: Y, 50g of metal ingot (99.9%) is used,
It was evaporated at an accelerating voltage of 5 KV and a filament current of 100 mA.

Cu: Alumina crucible wound with a tungsten filament was used as a resistance heating evaporation source, 10 g of metal Cu particles (2 to 3 mm, 99.9999%) were placed in the alumina crucible, and a current of 10 V and 30 A was applied to the filament. Evaporated.

Mg: 30 g of metal Mg (99.9%) was used, and this was put in a water-cooled crucible, and an electron beam was applied to the metal at an accelerating voltage of 5 KV and a filament current of 10 mA to evaporate.

〔The invention's effect〕

According to the method of the present invention, the surface of a single crystal YBa ₂ Cu ₃ O _7-X thin film in which the (001) plane is parallel to the film surface is only a few Å, which has been considered to be almost impossible in the past. It is possible to uniformly coat with an ultra thin film of MgO. Moreover, since the MgO ultra-thin film produced by applying this method is epitaxially grown with the (001) plane parallel to the film plane, the (001) plane becomes the film plane again. It is expected that a single crystal YBa ₂ Cu ₃ O _7-X thin film with excellent superconductivity will be epitaxially formed and used as a Josephson tunnel type device.

Furthermore, it is also possible to provide the above MgO ultrathin film as a substantially single-crystal one, in which case a high-quality superconducting thin film can be formed on the surface, so that it is expected to be used more as a Josephson tunnel type device. it can.

[Brief description of drawings]

FIG. 1 is a backscattered electron diffraction image of a thin film of YBa ₂ Cu ₃ O _7-X having a film thickness of 1000 Å obtained in Examples, and FIG. 2, FIG. 3, FIG.
Fig. 5 shows the backscattered electron diffraction images of the MgO thin film formed on the same thin film at the film thicknesses of 3Å, 6Å, 9Å and 21Å in order. It is a drawing substitute photograph to represent.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H01L 39/02 ZAA H01L 39/02 ZAAD 39/24 ZAA 39/24 ZAAJ // C04B 41/87 ZAA C04B 41/87 ZAAF (72) Inventor Kazuhito Hirata 103-3 Kamigamo Sakurai-cho, Kita-ku, Kyoto City, Kyoto Prefecture No.406 Maison Nakajima No. 406 (72) Inventor Naoshu Bando 8-15, Koyo-cho, Otsu City, Shiga Prefecture ( 56) References JP-A 64-54770 (JP, A) JP-A 1-205577 (JP, A)

Claims (3)

(57) [Claims] 1. A single crystal YBa ₂ Cu ₃ O _7-X thin film having a (001) plane parallel to the film surface, and a (001) plane parallel to the film surface A laminated thin film body characterized in that an ultrathin MgO film having a thickness of 10 Å or less is formed. 2. The laminated thin film body according to claim 1, wherein the ultrathin MgO film is formed as a single crystal. 3. Formed on a vapor deposition substrate in a vacuum vapor deposition tank,
Single crystal YBa ₂ Cu ₃ O _{7-X with} (001) plane parallel to the film plane
On the surface of the thin film, in the presence of a trace amount of oxygen, raise the temperature of the deposition substrate to 50
A method for producing a laminated thin film body, which comprises forming an MgO thin film by evaporating Mg at a rate of 2Å / sec or less at a temperature lower than 0 ° C.